Revert "tests: drivers: build_all: add fake serial device for modem tests"

This reverts commit 9e58a1e4.

This change is in conflict with commit 94f7ed35 ("drivers: serial:
add a dummy driver for vnd,serial"). As a result two equal serial
devices are defines, resulting in link error.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>